This invention relates to electronic phase detector circuits and more particularly to phase detector circuits suitable for use in phase-locked loops.
Phase-locked loop oscillators have many applications wherein a local oscillator must be synchronized to a received signal. Typically, a phase-locked loop includes a phase detecting device, a voltage controlled oscillator and a feedback circuit. The phase detecting device receives a reference data signal while simultaneously receiving a feedback data signal. In response thereto, the phase detecting device generates phase detection signals having an average amplitude indicating the difference in phase between the two data signals. The voltage controlled oscillator is coupled to receive the phase detection signal. In response thereto, the voltage controlled oscillator generates output signals having a frequency proportional to the magnitude of the phase detection signal. The output of the voltage controlled oscillator is used to form the feedback data signal through a feedback circuit.
Phase-locked loops are utilized, for example, to generate a control signal in synchronism with a reference signal. Synchronization between the control and reference signal is largely dependent upon the operational characteristics of the phase detector. This is because the phase detector controls the voltage controlled oscillator which changes its output frequency in response to the phase detection signal to correct errors indicated by the phase detection signal.
In the control circuitry of a solid state inverter or the like, it is often necessary to sense electrical waveforms in one circuit and then generate a control signal which accurately reflects a desired phase angle with respect to a reference signal. For example, in a variable speed constant frequency (VSCF) aircraft power system, it becomes necessary to force the output of a VSCF inverter into synchronism with an external power source. This permits momentary paralleling of the VSCF system with external power for non-interrupted power transfer. It is necessary to control the error between the VSCF output waveform and the external supply to small values. Any error will result in unbalanced real power flow. For lightly loaded conditions, this could mean that negative power would flow out of the VSCF inverter and positive power would flow out of the external supply during paralleled operation. If such a condition exists, even for as long as a tenth of a second, then the VSCF DC link voltage may rise to unacceptable levels. Therefore, an accurate and relatively fast phase-locked loop is required.
In VSCF systems, the output frequency is directly controlled by a voltage controlled oscillator. Further, any modulation of the voltage controlled oscillator input will cause frequency modulation of the VSCF output. In order to maintain good transient response, thereby minimizing DC link over-voltage transients, and to minimize frequency modulation of the VSCF output, it is necessary to have a relatively fast, low ripple phase detector in the phase-locked loop.
A common phase detector utilizes an exclusive OR gate to produce a phase detection error signal proportional to the difference in phase between a reference signal and a controlled signal. The exclusive OR phase detector produces a zero average output when the reference and controlled signal are at 90.degree. relative to each other. This results in a phase lock 90.degree. from the reference which can only be corrected by introducing an exactly 90.degree. phase shift on one of the signals. This adds complexity and uncertainty to the circuit and its performance. In addition, an exclusive OR phase detector has stable operating points, at +90.degree. and -90.degree., over a 360.degree. control range. Therefore, the system can lock 180.degree. out of phase from the desired location. Exclusive OR phase detectors also have a square wave output having a 50% duty cycle at phase-locked conditions. Therefore, a filter is needed between the phase detector and the voltage controlled oscillator to keep voltage controlled oscillator input voltage ripple and VSCF output frequency modulation to acceptable levels. Such a filter will hamper the transient response and stability of the phase-locked loop.
The present invention seeks to provide a phase detector circuit which has a single zero output point over the 360.degree. control range, zero output at zero phase error on the inputs and zero ripple at the zero output or null condition.